The present invention relates to measurement techniques. In particular this invention directs itself to a technique for highly localized measurements of complex microwave permittivity of materials and for near-field optical microscopy.
More in particular, the present invention relates to a probe for nondestructive determination of complex permittivity of a material based on a balanced mufti-conductor transmission line resonator which provides confinement of a probing field within a sharply defined sampling volume of the material under study to yield a localized determination of the material""s complex permittivity.
Furthermore, the present invention relates to fabrication of elongated dielectric support member based probes for localized measurements of complex permittivity of materials at frequencies from about 10 MHz to 100 GHz, where the fabrication process involves coating of an elongated dielectric support member with conducting material which is then processed to remove the conducting material from predetermined sides of the dielectric to yield a dielectric support member coated with a multi-conductor transmission line.
One of the main goals of near-field scanning microwave microscopy is to quantitatively measure a material""s complex microwave permittivity (dielectric constant and conductivity) with a high sensitivity of lateral and/or depth selectivity (i.e. to determine the material""s property over a small volume while ignoring the contribution of that volume""s surrounding environment). This is particularly important in measurements on complex structures, such as semiconductor devices or composite materials, where, for example, the permittivity of one line or layer must be determined without having knowledge of the properties of the neighboring lines or underlying layers.
In microwave microscopy a basic measurement is a determination of the reflection of a microwave signal from a probe positioned in close proximity to a sample. Phase and amplitude of the reflected signal may be determined directly by using a vector network analyzer or by determination of the resonant frequency and quality factor of a resonator coupled to the probe.
In many cases, the phase of the reflected signal correlates to a large extent with the real part of the sample permittivity, whereas magnitude is dominated by the imaginary part of the permittivity (i.e., the microwave absorption of the sample). Measurements of the microwave transmission from the probe through the sample are also possible, however, such an arrangement generally does not yield a localized determination of a sample""s complex permittivity.
The most typical approaches in microwave microscopy employ a coaxial probe geometry also referred to as apertureless probes, in which a central inner conductor (usually an STM tip) protrudes from one end of the probe and is tapered, as shown in FIG. 1. An alternative to the rotationally-symmetric arrangement of the coaxial probes are planar structures such as a co-planar wave-guide or a strip-line wave-guide. The tapered tip is used for concentrating the electric field around and/or underneath the tip apex which permits the probes to xe2x80x98imagexe2x80x99 features on the order of the tip apex curvature or less. This xe2x80x98imagingxe2x80x99 resolution, however, is not a quantitative measurement since the probe is averaging over a volume that is usually a few orders of magnitude larger (usually a few millimeters) than the tip apex curvature. While the field concentration around the tip apex is significant, there are also fields that extend over much larger distances. Such an apparatus yields an imaging resolution on the order of the diameter or radius of curvature of the central conductor tip.
It is obvious from considerations of classic electrodynamics that the volume of space over which an apparatus determines the electrical properties of a sample is determined not by the local dimensions of the central conductor tip alone, but rather by a length scale given by the separation between the central conductor tip and the ground (outer) conductor or shield, as shown in FIG. 1.
Therefore, in order to quantitatively determine the microwave properties of a material these properties must be devoid of non-uniformities on length scales of at least several times larger than the distance between the probe tip and the ground conductor while sufficient imaging contrast on length scales comparable to the radius of curvature of the tip may be achieved.
It is further obvious from considerations of classical electrodynamics that the separation between the probe and a sample affects the capacitance measured which is a function of the probe-sample separation and the electrical field distribution. Thus, it is important that the separation between the probe and the sample be measured in order to determine complex permittivity in a non-contact manner. Without accurate control of distance and a small volume of electrical field distribution, high lateral and/or depth selectivity and accurate quantitative results cannot be achieved with conventional technology.
Furthermore, the inherent unbalanced character of the exposed portion of the probe complicates the above-mentioned geometries due to the dipole-like current-flow in the surrounding area. The amount of radiation is critically dependent on the environment, i.e., the sample""s complex permittivity and the probe-to-sample distance which affects the amplitude of the reflected signal (reflection measurement) or quality factor of the resonator (resonant technique). The result is a potentially erroneous determination of the sample""s microwave absorption.
Conventional near-field microwave probes cannot be used for simultaneous near-field optical measurements and complex permittivity measurements due to the fact that the tapered fiber tip serves as a circular waveguide disadvantageously having a cut-off frequency for the optical and microwave radiation.
An object of the present invention is to provide a novel probe for the nondestructive determination of a sample""s complex permittivity based on a balanced multi-conductor transmission line resonator which is symmetric with respect to an exchange of signal between the conductors. This permits confinement of the probing field within the desired sampling volume to significantly reduce dependency of measurements on the sample volume""s environment.
It is another object of the present invention to provide a method for fabrication of a dielectric support member based (including a fiber-optic based) near-field microwave probe by means of coating an elongated dielectric support member with a conducting material which is further processed in subsequent technological steps to be removed from a predetermined number of sides of the dielectric support member resulting in the dielectric support member coated with a multi-conductor transmission line which can be used as a probe for complex permittivity measurements as well as providing a tip for near field optical microscopy.
In accordance with the principles herein presented, the present invention provides a novel probe for non-destructive measurements which includes a multi-conductor (preferably, a two-conductor) transmission line created on the dielectric support member. One end of the transmission line (also referred to herein as the xe2x80x9cprobing endxe2x80x9d) is brought into close proximity to the sample to be measured and may be tapered (or sharpened) to an end having a minimal spatial extent. A signal is fed through the transmission line toward the sample and a signal reflected from the sample is measured. The opposite end of the transmission line is connected to electronics for the determination of the reflected signal""s phase and magnitude. Measurements of the phase and magnitude of the reflected signal are broadband in frequency. Alternatively, if the probe is coupled to a resonator, the electronics then determine the resonant frequency and quality factor of the resonator which results in a measurement which is narrowband in frequency.
In this type of system, the diameter of the tip portion of the probe is in the range from 50 nm to 10 xcexcm, with the diameter of the elongated dielectric support member being in the range from 10 xcexcm to 10 mm. The conductive strips are separated one from the other around the dielectric support member by a distance not to exceed approximately 100 nm.
Each conductive strip is formed from a conductive material from a group which includes: metallic or superconducting materials, Au, Ag, Cu, Al, YBCO, Cr, W, Pt, Nb, etc., and mixtures thereof. Each conductive strip is formed upon a Cr, Ni, W, or Ta underlayer of 50-100 xc3x85 thickness directly deposited on the dielectric support member (or onto a cladding layer if the dielectric support member is fiber-optic based structure).
In accordance with the subject invention concept, such a dielectric support may be in the form of any of the following embodiments:
a) Optical fiber with exposed cladding, having bare (untapered) cladding diameter in the range of 10 xcexcm to 10 mm. The fiber may be formed from any insulating material that can be tapered by means of etching and/or heating/pulling, and has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.)
b) Dielectric rod (substantially circular in cross-section) from 10 xcexcm to 10 mm in outer diameter formed from any insulating material that may be tapered by means of etching and/or heating/pulling and has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.)
c) Dielectric tube (micropipette, etc.) from 10 xcexcm to 10 mm in outer diameter and appropriate inner diameter formed from any insulating material that can be tapered by means of etching and/or heating/pulling and has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.)
d) Dielectric tube (micropipette, etc.) from 10 xcexcm to 10 mm in outer diameter and appropriate inner diameter formed from any insulating material that may be tapered by means of etching and/or heating/pulling, and has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.) with an optical fiber inserted into the tube.
e) Dielectric bar of square, rectangular, pentagonal, hexagonal, octagonal, etc. cross-section with the cross-section linear dimensions from 10 xcexcm to 10 mm formed from any insulating material that may be tapered by means of etching and/or heating/pulling and has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.)
f) Multi-barrel dielectric tubing (with the number of barrels from 2 to 20) or Theta-tube formed from any insulator that may be tapered by means of etching and/or heating/pulling and which has a dielectric loss tangent xe2x89xa610xe2x88x921 at the operating frequency (e.g. quartz, sapphire, glass, etc.). One or more of the barrels may have an inserted optical fiber or metal wire.
The present invention is further directed to a method for manufacturing dielectric support member based probes, including fiber-based probes for near-field optical microscopy and/or complex permittivity measurements. For manufacturing the fiber-based probes, the method includes a preliminary step of:
removing an outer jacket from a fiber-optic wire of a predetermined length to expose a cladding layer surrounding a central optical fiber of the fiber-optic wire and
anisotropically depositing a 50-100 xc3x85 thick underlayer of Cr, Ni, W or Ta onto the cladding layer. For all other dielectric support member based probes, a 50-100 xc3x85 thick under layer of Cr, Ni, W or Ta is anisotropically deposited directly onto the dielectric support member.
The method further includes:
optionally removing the Cr, Ni, W or Ta underlayer between predetermined locations;
anisotropically depositing a conductive material on the Cr, Ni, W or Ta underlayer at predetermined locations; and
removing the conductive material between the predetermined locations, to form a plurality of electrically isolated conductive strips on the elongated dielectric support member (including fiber optic wire).
The conductive material and the Cr, Ni, W or Ta underlayer are deposited by one of any known conventional deposition techniques, for example, Pulsed Energy Deposition, Evaporation, Sputtering, Dipping, Focused Ion Beam Deposition, etc.
The conductive material and the Cr, Ni, W or Ta underlayer are removed by means of a number of material removal techniques, such as: Ion Beam Milling, Focused Ion Beam Milling, Chemical Etching, Mask-Less Photo-Lithography, etc.
The removal of the Cr, Ni, W or Ta underlayer may be omitted since it has been found that the method works with or without this step being incorporated.
For all of the aforesaid embodiments of the probe 10 of the present invention the procedure for fabrication of the probes is substantially the same and includes the following steps:
(i) The elongated dielectric support member is tapered down by means of chemical etching (e.g. using HF, etc.) and/or heating/pulling (e.g. using CO2 laser or heating filament based puller) thus forming a tip at the end with the apex curvature (or diameter) down to 10 nm;
(ii) The tapered dielectric support member is coated with a conducting material (e.g. Cu, Al, Ag, Au, etc.). Multi-layer coating may be used (e.g. Cr/Au) in this step.
(iii) The conducting material is removed from two or more sides to produce a multi-conductor transmission line that may be used as a probe for complex permittivity measurements.
For the embodiment (f), the procedure for manufacturing of the probe may include the following procedures:
(i) Two (or more) metallic wires (e.g. Cu, Al, Ag, Au, etc.) of appropriate diameter are inserted into the two (or more) different openings inside the multi-barrel or the capillary Theta-tube made of quartz;
(ii) The assembly obtained in (i) is tapered by means of heating/pulling (e.g. using CO 2 laser or heating filament based puller) thus forming a tip with the apex curvature (or diameter) down to 10 nm;
(iii) Optionally a tapered structure is uniformly coated with a conducting material (e.g. Cu, Al, Ag, Au, etc.) forming a shielded balanced transmission line. Additionally, a multi-layer coating may be used (e.g. Cr/Au). Such a structure permits construction of a balanced microwave transmission line and/or a resonator due to generally low microwave losses in quartz.
These and other novel features and advantages of this invention will be fully understood from the following detailed description of the accompanying Drawings.